Extended storage of bacteria is typically accomplished at very low temperatures (−80° C. and below). Not only have such very low temperatures been used to store bacteria, but they have also been used to store bacteria made competent for transformation (U.S. Pat. No. 4,981,797). However, problems are associated with the storage of bacteria and competent bacterial cells at higher temperatures (e.g., about −20° C. to about 4° C.). At these higher temperatures, bacteria and competent bacterial cells rapidly lose viability and transformation efficiency. Over a period of several months, the viable cell count and the transformation efficiency of such cells decreases by several orders of magnitude. Bacteria which are competent for transformation can only be stored at 4° C. for several days (Dagent et al., Gene 6:23-28 (1979)) or for a period of up to 16 days (Pope et al., Nucl. Acids Res. 24(3):536-537 (1996)). Thus, in order to maintain viability and competency, bacteria and competent bacterial cells have typically been stored at −80° C.
Stable storage of bacteria, and particularly competent cells, at temperatures higher than −80° C. is highly desirable, since many research laboratories may not have access to very low temperature storage. Moreover, the cost of storing bacteria at −80° C. is greater than at higher temperatures, and it is both difficult and expensive to transport bacterial cells at temperatures lower than −20° C.